Posted
by
timothy
on Wednesday March 23, 2005 @07:32PM
from the for-small-values-of-death dept.

Ant writes the "Solar Death Ray is made of 112 mirrors mounted on a platform 4 feet wide and 6 feet tall. Each mirror is a square roughly 3.5 inches on edge. All these mirrors focus the sun to a single spot 5 feet, 6 inches from the mirror platform. A wooden fork extends from the mirror base to the area near the focus and serves as a mounting point for Solar Death Ray targets. The mirror platform is mounted to the support frame on a pivot that allows the platform to be angled. The whole system is mounted on a set of wheels. The goal of the Web site was to show the results of the targeted items when the solar death ray was used."

the death ray is 4 feet mirror is 4 feet by 6 feet. It looks to have a bout 50% of its area covered with cheap mirrors, which I'll assume are about 80% refelective. that makes it about a square meter of effective reflectivity. the solar flux near the equator is about 1 kilowatt per sq meter. This is focused down to an area of about 6 inches square or about the size of a stove burner. A typical stove burner probably runs at about 1.5 KW. so basically this thing has the heat delivery of a burner. Actually a bit less since the object itself may be reflective over a large part of the spectrum. So call it maybe half a stove burner. Still plenty to fry plastic, your hand, or even start a fire.

I have this mental image of him trying to sneak up on the "enemy" to get that thing within 4 feet of them and then trying to get on the side of them away from the sun..."Behold the terrible power of the SUN! Hold still, please!"

It is only slower the first time a link it hit. The Coral network will download the content and cache it and it will be much faster from then. Also, the Coral network can handle a much higher load than this guys site can. Once the/. effect takes hold, the Coral network will be much fater. Try going back to the original link in my post. I bet it comes up very fast now since it has been cached in the Coral network. ; )

"Slashdot Death Ray is made of tens of thousands of geeks, most mounted on platforms approximately one to two feet high, and approximately 18 inches on each edge. Each geek focuses HTTP requests to a single web server at a distance ranging from tens to thousands of miles away. A web site is kept visible at all times on the geeks' computers and serves as mounting point for the URL of Slashdot Death Ray targets. The whole system is mounted on a large rock sphere. The goal of this summary is to show the results of the targeted website when the Slashdot Death Ray is used."

It's really only a "death ray" if you're really really tiny. Mythbusters [discovery.com] did a great job [kwc.org] of blowing the myth apart, with a much larger mirror array arranged in a proper fresnel configuration. It douldn't set fire to much of anything, even when they put gasoline on the target.

Given that he was able to set a rag on fire [solardeathray.com], I'm guessing that the Mythbusters team did something incorrectly regarding the focusing of their mirrors. And your link says they used a circular configuration which is only good in limited cases since the light is focused in a line (which isn't really focus) rather than a point. This was parabolic setup which is why he was able to melt plastic and set a rose on fire.

Ah, after some more reading, the myth itself that he did it with a circular configuration which they showed to be impossible. I guess they didn't know about parabolic reflections in those days. However, since a parabola is defined as the set of points equidistant between a point (the focus of the death ray) and a line (the infinitely far light of the sun reflecting off an imaginary flat mirror) this means that all the tangents of a parabolic curve (the flat mirrors in this case) will always cause the light

They were attempting to replicate what Greeks could have reasonably done with the technology they had available. The myth isn't that you can use a bunch of mirrors to set things on fire. The myth is that Greeks 2500 years ago were able fire ships some distance away in a harbor. They wouldn't have been using any sort parabolic mirror and even a concave one of any reflectivity at all would be a serious stretch. The Mythbusters did a decent job of showing that the ancient Greeks probably didn't have sufficient mastery of optics to make a practical sunlight weapon.

We don't know what technology the Greeks had; we know very little about them. What we do know indicates that this was possible.

They certainly knew geometry and optics. What they didn't know was glass. Crystal lenses have been discovered all over the Mediterranean.

Discovering how a parabolic array (lots of flat mirrors lined up along the curve of a parabola) focuses light is something that Archimedes could have confirmed - all it takes is an inquistive mind and observation. Getting a few hundred soldiers to position shields correctly would have been fairly trivial.

Why wouldn't it have become a popular weapon? It's not reliable enough. You need to have lots of mirrors, room to set them up in a parabolic curve, lots of bright sunlight, and a relatively slow moving target that will cross a known point at the right time. It's not easy but it would be possible.

Consider that triremes didn't usually sail at night - an invading fleet might well have pulled up not far from the harbor for the night, with intent to sail in an hour or two after dawn.

A parabolic array only matters when you are trying to focus a signal. The Greeks were only interested in energy, and had no concern for phases. Therefore they don't need anything other than clear line of sight to the target for everyone. Each person just has to figure out which of the (many) bright spots is the one they control, and keep that more or less on the target. So long as the average energy reaching the target spot is enough it doesn't matter if many are not on target at any particular moment.

They certainly knew geometry and optics. What they didn't know was glass. Crystal lenses have been discovered all over the Mediterranean.

Ancient Greece isn't my specialty -- that would be Egypt -- but I know that by the time the Greeks were trading with the Egyptians, blown glass artifacts start showing up, initially as imports, and later as domestic products. The Egyptians had been making cast-glass jewelry for some time before that. I rather doubt they knew how to make optical-grade glass, though. That

Apparently it worked when it was tried in 1973 [mlahanas.de] (see middle of page).

A Greek scientist, Dr. ioannis Sakkas, curious about whether Archimedes could really have used a "burning glass" to destroy the Roman fleet in 212 BC lined up nearly 60 Greek sailors, each holding an oblong mirror tipped to catch the Sun's rays and direct them at a wooden ship 160 feet away. The ship caught fire at once.....Sakkas said after the experiment there was no doubt in his mind the great inventor could have used bronze mirrors to scuttle the Romans

Mythbusters is far too quick to "bust" myths that are actually true to some extent. They make a couple of (usually poorly designed) tries to replicate the circumstances, and then when their small number of tests fail they declare the myth "busted".

This is a perfect example. Mythbusters claims to have "busted" the solar death ray myth, yet the guys in this article were successful in lighting shop rags, pairs of old jeans, boardgames, etc on fire, and have pictures to prove it.

No, they claim to have busted the myth that the ancient Greeks set ships on fire hundreds of feet away. Setting an object on fire with a mirror three or four feet away is a vastly different feat from setting a ship on fire 100 feet away.

Like hell Archimedes didn't set ships on fire with mirrors! We're talking about a guy who built a giant mechanical arm to tip over ships in the harbor in the 3rd century BC! I think burning ships with mirrors would be easier than that!

Ummmm, shields aren't concave, they're convex. I suppose they could be turned around, but then the handles and stuff counteract the effectiveness of the "focusing". Also, focusing only really helps at near the focal length. Beyond twice the focal length it should disperse rays that started as parallel.

The Mythbusters did a horrible job at busting that myth. The myth was that Archimedes had the soldiers uses their shields as mirrors to focus light on the ships. Mythbusters almost did a great job, but forgot one important thing. A shield is concave, which has the amazing property of focusing light. The Mytbusters used flat mirrors.

That depends greatly on the era and the shield, flat ones are not unheard of. Typically the concave side (of a curved shield) is where the handles are located, so it's unlikel

Mythbusters almost did a great job, but forgot one important thing. A shield is concave, which has the amazing property of focusing light. The Mytbusters used flat mirrors.

If you want to burn a ship that's several hundred meters away with a reflection from the sun, it doesn't matter very much whether the mirrors have exactly the right curvature or are flat. Even a perfect curved mirror would create a perfect image of the sun the diameter of which depends on the distance between the mirror and the image. At

Wonder how much juice you could generate if you were to mount a stirling engine [howstuffworks.com] at the end of this sucker. Seems like it'd be a lot cheaper/easier to implement than normal high efficiancy solar cells if you could work out a reasonable and reliable sun tracking system.

Wonder how much juice you could generate if you were to mount a stirling engine at the end of this sucker. Seems like it'd be a lot cheaper/easier to implement than normal high efficiancy solar cells if you could work out a reasonable and reliable sun tracking system.

Warning!The slashdot is bright. Don't look at the slashdot or you will damage your eyes. Anything that focuses the slashdot will only make it more dangerous. The Slashdot.org is dangerous. Don't build one.I'm surprised I haven't burnt or blinded myself yet. The fumes from molten trolls can't be good either. Don't play with flames.

THIS [caltech.edu] is a solar death ray: 10 metres of high-precision parabolic polished aluminium. (And there are bigger ones out there in the world too.)

I've observed there. Because it is radio astronomy, we could observe before sunset and after sunrise, but for some reason we had strict instructions to never let the sun fall on the dish. (That includes the back, but that was to do with thermal distortion of the dish, rather than frying the focus.)

I also used my HP48SX calculator (running a terminal emulator) to command the telescope to slew. Because of this, I claim the CSO as world's the largest and most expensive peripheral for a pocket calculator.

I also used my HP48SX calculator (running a terminal emulator) to command the telescope to slew. Because of this, I claim the CSO as world's the largest and most expensive peripheral for a pocket calculator.

You are such a nerd. We will have to make you king of the winter carnival.

10 metres of high-precision parabolic polished aluminium

Why aluminum? Is it the most reflective substance on earth?

we had strict instructions to never let the sun fall on the dish

No matter where you point it, you are pointing it somewhere.

And make sure to not leave it pointing in the direction of the only all-black fraternity house on campus. That could start up those nasty black versus nerd wars again. Instead, point it at the Sigma Chi house, those bastards are always burning down their own house... nobody will suspect anything.

polished aluminum is used because its sub-mm but not extremely sub-mm (ie, optical).

its been awhile, so i forget the exact formula, but basically your surface can be as rough as some fraction of the wavelength you're trying to focus. Hence, wide waves can use dirty and/or rough surfaces (such as arecibo, which is just a hole in the ground and some perforated aluminum panels) and still work just fine, even when soiled as a huge bowl in the ground is bound to become.

Larry Niven invented the "flashmob" years ago. Now, it looks like someone has come up with something similar to his Ringworld "Sunflowers", which consisted of petal-ringed mirrors which could focus on prey and turn it into ash fertilizer.

The parabolic reflector gaves at the focal point a maximum flux of 1000 W/cm2. The experimentations takes place at the focal zone (18 m in front of the paraboloid. The range of available temperature is from 800 to 2500 C (the maximum reachable temperature is 3800 C) for a maximum thermal power of 1000 kW.(Did someone just say holy fucking shit?) Picture of the Odeillo Solar Furnace [latrobe.edu.au]

"Slashdot Death Ray is made of 1 website and no mirrors of the target site mounted on a platform of linux, perl, and horribly mangled html. Each user is a square (ed. note: update to modern parlance, ie "geek"). All these mirrors focus the slashdot to a single web server. A wooden fork is stuck into the web server after it melts to signify that it is "done". The mirror platform is often asked for and often denied by CmdrTaco, who mounted his stock answer on an FAQ somewhere. The whole system is mounted on a stack of open protocols dating back to the early days of DARPA. The goal of the Web site was to show the results of the targeted items when the slashdot death ray was used."

I always wanted to round up a kindergarten class worth of children to act as my solar death squad. Here's how it would work:

1. Give each kid their own shiny little mirror with a post-it note stuck on it to block the shiny part2. One at a time, have them remove the post-it, aim their mirror to reflect the sun upon some point, then re-post-it.3. Once everyone is aimed (30 kids or so), have them all remove their post-its at once, instantly creating a plasma-hot ball of fire at the point of focus, incinerating your enemies with the might of a kindergarten class.

Has anyone else had this idea too, or am I the only weirdo around here?

How about a board of little mirros on motor-controlled pivots. If you point the board directly at the sun, with a single command from a controlling computer you could redirect all of the mirrors to any point in your firing arc at any distance.

Sorta, anyway. This is a link to the UNLV Solar Project, a project playing with new ideas in solar energy such as focus the light to a specific point (roughly, of course) to increase the uptake of energy by the receptors. I drive by these bad boys everyday. UNLV Solar [unlv.edu]

How about blinding the sailors on board, who then run around in a panic and knock over the pot of charcoals used for igniting the flaming arrows? Carcoals ignite the ship's deck instead, or someone's clothes, the fire spreads, voila. No more battleship.

No, it was a laser... very intense highly colimated (parallel) light. This site is about a parabolic reflector, which makes the light converge on a small area. Lasers can target any point in line with the beam. With a parabolic reflector, the light gets weaker (less concentrated) as you move past the focus. Beyond the distance between the reflector and the focal point the light is weaker than the origional light. Of course this is a faceted reflector, so the light isn't really weaker, it's just less and less likely that any point on a plane parallel to the relector will be illuminated the further away from the focal point you are.

Magnification does nothing. It is light intensity that counts. You may use a magnifier to focus the light from a larger area to a smaller area - you don't magnify it. The mirrors do the same thing.

Proof: Take a microscope and set it to 500X. Point the objective at the sun. Do you death rays spewing from the eyepiece? (Answer: no). To find out why, read the first paragraph or ask someone that *really* knows. (Hopefully someone that took some optics (physics) or astronomy)

Take a microscope and set it to 500X. Point the objective at the sun. Do you death rays spewing from the eyepiece? (Answer: no).

A while back I was at an observatory [cmnh.org] and the guy in charge said never to point a telescope at the sun. To demonstrate, he turned the telescope (10.5" refractor) toward the sun. We could see a beam of bright light coming out the eyepiece. He put a piece of paper in the middle of the light and it ignited into flames almost instantly.

What caused the ignition was that about 250watts of light was concentrated on a small spot on the paper. You would have gotten the same effect with a 10.5" parabolic mirror. Actually, if it had been a reflecting telescope, it would have used a 10.5" parabolic mirror:)

Exactly. In order to get the same power output from a completely magnification based setup, you'd need a magnifier with area about equal to the area of all the mirrors put together. The only really feasible way to do this is to use a fresnel lens, a normal lens would either be much too thick in the middle (because the thickness is proportional to the radius, and we're talking a pretty good sized lens) or else the focal point of the lens would be pretty far away...